1. Field of the Invention
The present invention relates to a semiconductor device, more particularly, to a semiconductor device including a semiconductor chip in a cavity on a ceramic base and a ceramic cap sealing the cavity.
2. Description of the Related Art
Heretofore, semiconductor chips have been emplaced in cavities of lead bases or leadless bases and sealed with caps. Generally, these caps have been made of metal for reasons of workability, solderability, cost, etc. For face-down type semiconductor devices, in which lead pins are provided at the face side of the base, for example, for provision of cooling fins at the rear of the base, the cap is preferably made of ceramic. This is because, when lead pins of such a semiconductor device are dipped in a molten solder bath to treat the outer surface of the lead pins or when such a semiconductor device is mounted onto a printed circuit board by soldering, adherence of solder onto the cap or formation of short-circuits between the lead pins and the cap may occur if the cap is made of metal.
In a semiconductor device including a ceramic base and a ceramic cap, the cap is bonded to the base by metallizing the two in the regions of contact and soldering them in the metallized regions. Soldering is carried out by inserting a thin ring pattern of solder between the ceramic base and cap, melting the solder while simultaneously pressing the cap against the base, and then cooling. The pressing of the cap is effected to prevent formation of voids in the soldering portion between the base and the cap since a ceramic cap is often somewhat warped.
However, we found that after soldering, excess solder flowed or flew from the soldering portion into the cavity of the base and outside the base and the cap, resulting in short-circuits and other defects of the semiconductor device. Such excess solder was often found at the corners of the cavity of the base. Further, there are problems in sealability, since solder did not cover the side surfaces of the cap or did not form a meniscus at the corners formed by the side surfaces of the cap and the top surface of the base due to absence of metallization on the side surfaces of the cap.
Japanese Unexamined Patent Publication (Kokai) No. 55-148445 disclosed formation of a separate pattern of metallization in addition to a pattern of metallization for soldering on an inner surface of a ceramic cap. The separate metallization pattern is formed at a portion spaced from and more inside than the metallization pattern for soldering on the surface of the cap facing the base. This separate metallization pattern is, however, not that effective for preventing the flow or flying out of excess solder toward the outside of the cap and the base and into a cavity of the base. Also, the problem of sealability between the cap and the base is not solved here.